I want to write a letter regarding a
consumer complaint I have. What should I include in the
letter?

Your letter should include the following:

A complete description of the problem, the product
number, and the date and place of purchase.

A description of what happened at the local level when
a complaint was made (include dates and the person with
whom you spoke).

A statement of what you would consider reasonable to
resolve the problem.

The action intended if satisfactory redress is not
obtained.

A reasonable deadline for them respond to your letter
(3 weeks is a reasonable time).

Copies of relevant documents (keep all originals).

Indication that copies of the letter are being sent to
higher offices in the company and/or government offices
(be sure to keep a copy for your files).

Be direct but not threatening. You can demand a solution
to your problem while keeping your requests reasonable,
which will more likely ensure that you will receive some
sort of resolution to your problem.